1. Field of the Invention
The present invention relates to adaptive audio equalizer apparatuses in which effects on acoustic signals due to transmission characteristics of the signal path are automatically compensated. More particularly, the present invention relates to an adaptive audio equalizer apparatus in which ringing is prevented, and to a method of determining a filter coefficient for the adaptive audio equalizer apparatus.
2. Description of the Related Art
When sound such as music is reproduced by an audio system, the sound waves undergo reflection and absorption during transmission over the signal path. The signal path has unique transmission characteristics, which alter the magnitude and phase of the sound waves differently for various frequency components, thereby producing degenerated sound. Thus, it is desired that the magnitude and phase of the audio signals be compensated on a frequency basis in accordance with the transmission characteristics of the signal path.
Adaptive audio equalizer apparatuses have been known as devices for automatically compensating, on a frequency basis, for the effects of the signal path on the magnitude and the phase of acoustic waves. FIG. 5 is a block diagram of a conventional adaptive audio equalizer apparatus. The adaptive audio equalizer apparatus includes an input terminal 31, a target response output unit 32, an adaptive filter 33, a speaker 34, a microphone 35, a calculation unit 36, and an FIR (Finite Impulse Response) filter 37. The adaptive filter 33 includes an FIR filter unit 33a, and a filter coefficient setting unit 33b for automatically setting, in accordance with the LMS (Least Mean Square) adaptation algorithm, a filter coefficient used in the FIR filter unit 33a. 
The input terminal 31 receives an audio signal from a CD player, FM tuner, etc. The target response output unit 32 delays the audio signal that is input via the input terminal 31 for a predetermined period, and outputs a target response signal. The adaptive filter 33 adjusts on a frequency basis the magnitude and phase of the signal that is input via the input terminal 31. The speaker 34 outputs the signal fed from the adaptive filter 33 to the acoustic space to produce the corresponding sound.
The microphone 35 is provided at a listening point in the acoustic space, detects the sound waves produced from the speaker 34, and converts the detected sound waves into an electric signal. The calculation unit 36 calculates the difference between the target response signal that is output from the target response output unit 32 and the signal that is output from the microphone 35, and provides the result as an error signal. The FIR filter 37 approximates the transmission characteristics of the acoustic space C with respect to the signal path from the speaker 34 to the microphone 35, compensates the audio signal in accordance with the transmission characteristics, and provides the output to the adaptive filter 33.
The sound waves that are output from the speaker 34 to the acoustic space are altered in accordance with the transmission characteristics of the signal path while travelling to the microphone 35. In order to match the signal that is output from the microphone 35 and the target response signal, the adaptive filter 33 continually updates the filter coefficient so as to minimize the power of the error signal. The adaptive filter 33 adjusts the magnitude and the phase of the audio signal on a frequency basis in accordance with the filter coefficient. Accordingly, effects of the signal path on the audio signal are compensated, so that sound of desired quality is produced.
In the adaptive audio equalizer apparatus as described above, if the frequency characteristics of the signal detected by the microphone 35 exhibit a dip as shown in FIG. 6A, the adaptive filter 33, with regard to the filter characteristics (frequency-gain characteristics) thereof, forms a peak corresponding to the frequency of the dip, as shown in FIG. 6B, in order to achieve flat frequency characteristics of the signal that is output from the microphone 35.
However, when a very high peak is required in the frequency characteristics of the filter, the peak value may not be in a range that the construction of the filter allows. Furthermore, it is often the case that the dip is not present in the frequency characteristics of the sound waves at positions other than where the microphone 35 is disposed. In that case, over-compensated sound with an excessive peak in the frequency characteristics is generated, causing what is called ringing, which sounds uncomfortable to the listener.
Accordingly, it is an object of the present invention to provide an adaptive audio equalizer and a method of determining a filter coefficient therefor, which serves to produce desired sound through compensation in accordance with the transmission characteristics of the signal path, and to inhibit excessive compensation and therefore to inhibit ringing.
To this end, according to one aspect of the present invention, there is provided an adaptive audio equalizer apparatus having an adaptive filter including a finite impulse response filter unit, and a filter coefficient setting unit for determining a filter coefficient for the finite impulse response filter unit, the adaptive filter being for compensating an input signal that is input to the finite impulse response filter unit; a target response output unit for providing a target response signal; a speaker to which a signal that is output from the adaptive filter is supplied; a microphone for detecting acoustic waves produced by the speaker and converting them into an electric signal; a first calculation unit for calculating a difference between a signal that is output from the microphone and the target response signal, and for providing the result as an error signal; a delaying and multiplying unit for delaying an output of the adaptive filter and multiplying it by a scaling factor; and a second calculation unit for adding an output of the delaying and multiplying unit and the error signal, and for providing the result to the filter coefficient setting unit of the adaptive filter.
Preferably, the adaptive filter updates the filter coefficient so as to minimize the power of the signal supplied to the filter coefficient setting unit.
It is also preferable that the signal that is input to the finite impulse response filter unit is an aperiodic signal such as a white noise signal, so that there is no correlation between the signal that is input to the finite impulse response filter unit and the signal that is input to the filter coefficient setting unit.
According to another aspect of the present invention, there is provided an adaptive audio equalizer apparatus having an adaptive filter including a finite impulse response filter unit, and a filter coefficient setting unit for determining a filter coefficient for the finite impulse response filter unit, the adaptive filter being for compensating an input signal that is input to the finite impulse response filter unit; a target response output unit for providing a target response signal; a speaker to which a signal output from the adaptive filter is supplied; a microphone for detecting acoustic waves output from the speaker and converting them into an electric signal; a first calculation unit for calculating a difference between a signal that is output from the microphone and the target response signal, and for providing the result as an error signal; a noise signal source for generating a particular noise signal having no correlation to the input signal; a finite impulse response filter, in which the same coefficient as in the adaptive filter is set, for compensating the particular noise signal input from the noise signal source; a multiplying unit for multiplying an output of the finite impulse response filter by a scaling factor; and a second calculation unit for adding an output of the multiplying unit and the error signal, and for providing the result to the filter coefficient setting unit of the adaptive filter.
Accordingly, when there is an excessive peak at a frequency in the filter characteristics of the adaptive filter, the filter coefficient setting unit recognizes a large error at that frequency, thereby updating the filter coefficient so as to reduce the peak. Thus, an excessive peak in the filter characteristics is inhibited, thereby inhibiting ringing which sounds uncomfortable to the listener.